Semiconductor devices based on silicon carbide (SiC) as base material are continuously developed to be used in high-temperature contexts, high-power applications and under conditions involving a high radiation, under which circumstances conventional semiconductors cannot function satisfactorily. Estimations indicate that SiC transistors of power MOSFET type and diode rectifiers of SiC could operate over larger voltage and temperature intervals, for example up to 650.degree.-800.degree. C., and exhibit better breaker properties with lower losses and higher frequencies, and still be 20 times smaller in volume than corresponding silicon components. These improvements are due to the inherent advantageous material properties which silicon carbide possesses in relation to silicon, as for example a higher breakdown field (up to 10 times higher than silicon), a higher thermal conductivity (more than 3 times higher than silicon), and a higher energy band gap (2.9 eV for 6H--SiC, one of the crystal structures for SiC).
Since the silicon-carbide semiconductor technique is relatively young and in many respects non-optimized, there are many critical manufacturing problems which require a solution before fully useful SiC power semiconductors may be realized experimentally and manufacture in larger quantities may be carried out. This is especially true of components intended for high-power and high-voltage applications. Difficulties requiring a solution are that the background doping concentration for the voltage-absorbing layer in the component must be reduced for one single component to be able to withstand voltages of several kilovolts, that the surface passivation technique of the silicon carbide must be optimized, and that the quantity of critical defects in the silicon carbide material must be reduced if, for example, heavy-current components with large areas are to be manufactured. Other areas which require development are, for example, methods for manufacturing good ohmic contacts for the material, methods for doping with, for example, implantation, and process techniques for, for example, etching, etc.
Manufacturing high-volt diodes in 6H--SiC with epitaxially created p-n junctions and Schottky junctions has been carried out for experimental purposes (see, e.g., M. Bhatnagar and B. J. Baliga, IEEE Trans. Electron Devices, Vol. 40, No. 3, pp 645-655, March 1993, or P. G. Neudeck, D. J. Larkin, J. A. Powell, L. G. Matus and C. S. Salupu, Appl. Phys. Lett., Vol. 64, No. 11, 14 Mar. 1994, pp 1386-1388). Some of the problems described above have been solved, such as among other things the reduction of the doping concentration, whereby the first 2000 V silicon carbide diodes ever have been reported. This has been realizable because of the progress of development in recent years for manufacturing substrate materials in silicon carbide.
However, no simple method for surface-passivating silicon carbide useful for restructuring the surface of a silicon carbide material for obtaining a high-resistance layer from the surface down to a desired depth is known.
Another difficulty to master during manufacture of high-volt diodes or other semiconductor devices with a voltage-absorbing p-n junction is to achieve a suitable termination of the edge of the p-n junction. The electric field across the p-n junction at the edge of thereof is very large when a high reverse voltage is applied across the p-n junction. This problem has not been solved in the above-mentioned known diodes. Many of the problems which remain to be solved during development of semiconductor devices in SiC bear a strong resemblance to those which occurred when the corresponding silicon components were introduced. Still, the same techniques cannot be applied for solution of the specific problems relating to the manufacture of silicon carbide components as with the currently known solutions for the corresponding production of silicon components. As an example, it may be mentioned that doping by diffusion in SiC is very difficult to carry out, since diffusion coefficients are negligible below about 2270K. Also ion implantation of dopants, which is a common technique when manufacturing silicon components, is difficult to master and not developed for silicon carbide.
A high voltage in the reverse direction with the ensuing strong electric field at the edge of a p-n junction gives a great risk of breakdown or flashover at the edge of the p-n junction. Where the p-n junction emerges at the surface of a component, an increase of the electric field arises compared with the situation at the p-n junction further inside the component. This has to do with a change from more homogeneous conditions inside the component to the abrupt step out of the material at the surface. This fact makes it important to reduce the field at the surface and to passivate the surface. Combined with the surface of a silicon comonent being passivated, measures are also taken to equalize the field at the surface by, for example, influencing how the p-n junction emerges to the surface. In connection with power components, for example, lapping (grinding) of the surface is performed at a certain angle through the p-n junction in order thus to equalize the field. In these contexts, the designation negative edge angle is used for designating that a wafer with a p-n junction has an increasing area when proceeding from a layer with a high doping concentration to a layer with a low doping concentration (e.g, from a p-region with a high to an n-region with a low doping concentration). Correspondingly, a positive edge angle designates that a p-n junction has a decreasing area in a direction towards the layer with a low doping concentration (e.g. in a direction from a p-region with a high to an n-region with a low doping concentration). For example in FIG. 3, angle .alpha. should be between 0.degree. and 90.degree. to form a positive edge angle. One way of reducing the electric field at the edge of a p-n junction is to design the p-n junction with a positive edge angle, whereby this edge angle in the case of known semiconductor materials is achieved by means of grinding of the edge, a method which does not occur at all in a silicon carbide component.
A further method of reducing the field concentration at the edge of the p-n junction is to gradually reduce the p-doping up towards the surface in a ring peripherally around the p-n junction (so-called junction termination extension) for reducing the field at the surface. These methods known from the silicon technique are difficult to apply to materials in SiC because the material is hard, doping with diffusion is difficult to carry out, etc.
For terminating a p-n junction with silicon as base material, it is known, for example, to provide the edge of the p-n junction with a semi-insulating layer with a certain desired, very small conductivity. This layer, when a high voltage in the reverse direction is applied to the p-n junction, carries a weak current in the reverse direction via this semi-insulating layer, whereby the electric field at the edge of the p-n junction decreases and the risk of voltage breakdown is considerably reduced. The patent document SE 9400482 shows a method of edge terminating and passivating a p-n junction designed in an SiC base material. The present invention describes an additional solution to this problem.
It is further known to achieve edge termination with the aid of argon ion implantation in a Schottky diode (IEEE Electron Device Letters, Vol. 15, No. 10, Oct.94, Dev Alok et al). This publication, however, only mentions that a thin layer at the surface of a Schottky diode may be passivated, nothing is mentioned about edge termination of a p-n junction consisting of at least two different silicon carbide layers.